Galaxies in Hiding

Sometimes astronomy is a bit like fishing: patience is the cardinal virtue. A couple of years ago astronomers trained the Hubble Space Telescope on a fairly empty patch of sky and left it there for ten days, trying to catch whatever photons straggled in. The result was the Hubble Deep Field, a series of images that doubled astronomers' estimates of the number of galaxies in the universe to at least 50 billion.

Now researchers in Hawaii have done something similar. Using a new instrument that can peer through the dust that obscures many galaxies, Amy Barger and her colleagues at the University of Hawaii built up images of small parts of the sky over the course of two weeks. They've uncovered evidence of a population of never-before-seen galaxies--so many, in fact, that taken together they shine as brightly as all the rest of the known galaxies in the universe.

The new instrument on the James Clerk Maxwell Telescope on Mauna Kea allows astronomers to map areas of the sky in wavelengths some 1,000 times longer than visible light. This is useful for finding galaxies because while large clouds of dust will scatter or absorb visible light, they transform light emitted from the galaxies into infrared light. Observing just two small patches of the sky, Barger and another astronomer found three hidden galaxies. If this tally is typical of the rest of the sky--and Barger sees no reason why it shouldn't be--then there are at least 40 million such galaxies in the universe. What's more, Barger says, these galaxies seem to emit 100 times as much energy as the galaxies we know about.

The unusual brightness might be due to bursts of star formation, says Barger. She'd like to point the Hubble at these galaxies to detect visible light coming from them, which would help her determine their distance--and their age. "The problem is that there's very little visible light making it through," Barger says. If the galaxies turn out to be very old, a distinct possibility, it may mean that astronomers will have to revise not only their count of the number of galaxies in the universe but the history of galaxies as well.